There are two sample times, T, used in process controller design and tuning.
One is the control loop sample time that specifies how often the controller samples the measured process variable (PV) and then computes and transmits a new controller output (CO) signal.
The other is the rate at which CO and PV data are sampled and recorded during a bump test of our process. Bump test data is used to design and tune our controller prior to implementation.
In both cases, sampling too slow will have a negative impact on controller performance. Sampling faster will not necessarily provide better performance, but it is a safer direction to move if we have any doubts.
Fast and slow are relative terms defined by the process time constant, Tp. Best practice for both control loop sample time and bump test data collection are the same:
| Best Practice: Sample time should be 10 times per process time constant or faster (T ≤ 0.1Tp). |
We explore this “best practice” rule in a detailed study here. This study employs some fairly advanced concepts, so it is placed further down in the Table of Contents.
Yet perhaps we can gain an appreciation for how sample time impacts controller design and tuning with this thought experiment:
Suppose you see me standing on your left. You close your eyes for a time, open them, and now I am standing on your right. Do you know how long I have been at my new spot? Did I just arrive or have I been there for a while? What path did I take to get there? Did I move around in front or in back of you? Maybe I even jumped over you?
Now suppose your challenge is to keep your hands at your side until I pass by, and just as I do, you are to reach out and touch me. What are your chances with your eyes closed (and loud music is playing so you cannot hear me)?
Now lets say you are permitted to blink open your eyes briefly once per minute. Do you think you will have a better chance of touching me? How about blinking once every ten seconds? Clearly, as you start blinking say, two or three times a second, the task of touching me becomes easy. That’s because you are sampling fast enough to see my “process” behavior fully and completely.
Based on this thought experiment, sampling too slow is problematic and sampling faster is generally better.
Keep in mind the “T ≤ 0.1Tp” rule as we study PID control. This applies both to sampling during data collection, and the “measure and act” loop sample time when we implement our controller.